The S-content-dependent lattice structure evolution and bandgap modulation in quaternary MgZnOS alloy films

Abstract

Bandgap engineering of ZnO by alloy formation is of great importance for its application in modern optoelectronic devices. Herein, Mg and S co-substituted quaternary Mg0.12Zn0.88O1−y S y (MgZnOS) alloy films with various S content were grown on c-plane sapphire by pulsed laser deposition using a Mg0.12Zn0.88O0.18S0.82 ceramic target under various O2 partial pressures. The S-content-dependent phase structure evolution and S solubility limits in single-phase MgZnOS alloys were determined, and the correlation of lattice constants and band gap with the S content of the single-phase MgZnOS was quantitatively established. It turns out that the MgZnOS films grow quasi-epitaxially on c-sapphire with a wurtzite structure, which evolves intricately with varying S content. The S-rich MgZnOS films assume both lattice constants and in-plane orientation similar to those of ZnS. With decreasing S content, the alloy lattice first contracts along the out-of-plane direction, then shrinks in-plane, and partly re-orients by 30° in-plane, eventually approaching ZnO for the O-rich MgZnOS films. S content (y S) achieved in the single-phase O-rich and S-rich Mg0.12Zn0.88O1−y S y films is y S ⩽ 0.33 and y S ⩾ 0.67, respectively, far beyond the S solubility limits in the counterpart ternary ZnOS. While phase separation of MgZnO and MgZnS takes place in the films with S content between 0.34–0.65, in-plane domain separation with mutual rotation by 30° occurs in the O-rich single-phase MgZnOS films with y S ⩽ 0.08. Moreover, for the O-rich single-phase MgZnOS films, the lattice constant c expands linearly while a remains almost invariant with increasing S content. The band gap of MgZnOS is nonlinearly adjustable in the range of 3.13–3.66 eV, with a bowing parameter (∼1.89 eV) smaller than that of ZnOS (∼3.0 eV). The S-content-dependent evolutions of both lattice constants and band gap of MgZnOS differ distinctly from those of ZnOS, indicating bright prospects for synergistic Mg and S co-substitution in the effective modulation of both structure and band gap of ZnO to meet specific applications.